DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
Commento - A fast, bloat-free comments system to foster discussion on your website
utterances - A lightweight comments widget built on GitHub issues.
IntenseDebate - IntenseDebate's comment system enhances and encourages conversation on your blog or website.
Hyvor Talk - The Best Commenting Platform For Your Website
Juvia - A commenting server similar to Disqus and IntenseDebate